The present invention relates to thermal shields for use in gas turbine engines and, more particularly, to a thermal shield for thermally insulating a turbine casing from high temperature fluid flow.
Thermal shields are used in gas turbine engines to thermally isolate particular structures from an active heat transfer environment. The effectiveness of these shields, which are a combination of a metal foil backing enclosing an insulation type blanket next to the structure, is directly dependent upon having no gaps or channels between the blanket and the structure and upon the blankets retaining their original shape. Gaps or channels between the blanket and the structure have an inherent "flow leak". Leaks have an associated flow velocity which can generate a significant heat transfer coefficient. The prior art has encountered problems in end sealing of these thermal or heat shields. Gaps between the heat shield and turbine structure allow heated fluid to flow over the structure which the heat shield is intended to protect. Thermal distortions and part-to-part tolerancing compromise the ability of the heat shield to act as an effective seal. Most heat shields used in standard turbine/compressor design applications, have an "inside" radial fit-up. This radial fit-up cannot be controlled effectively during engine transient operation. In addition, vibration of the engine structure can cause the fibrous insulation blanket to deteriorate and lose shape thereby providing a flow path between the blanket and the structure insulated by the blanket.
Thus, a need exists for an effective heat shield mechanism which alleviates the effect of gap leaks, which better isolates a turbine structure from hot fluid flow, and which has good dimensional stability under engine operating conditions.